Silk Fibroin Separators: A Step Toward Lithium-Ion Batteries with

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5385−5394

Silk Fibroin Separators: A Step Toward Lithium-Ion Batteries with Enhanced Sustainability Rui F.P. Pereira,*,†,‡ Ricardo Brito-Pereira,§ Renato Gonçalves,†,§ Marco P. Silva,§,∥ Carlos M. Costa,*,†,§ Maria Manuela Silva,† Verónica de Zea Bermudez,‡,⊥ and Senentxu Lanceros-Méndez*,#,¶

ACS Appl. Mater. Interfaces 2018.10:5385-5394. Downloaded from pubs.acs.org by WESTERN UNIV on 09/20/18. For personal use only.

†

Centro de Química and §Centro de Física, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal ‡ Departamento de Química, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal ∥ C-MASTCentre for Mechanical and Aerospace Science and Technologies, Universidade da Beira Interior, 6200-001 Covilhã, Portugal ⊥ CQ-VR Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal # BCMaterials, Parque Científico y Tecnológico de Bizkaia, 48160 Derio, Spain ¶ IKERBASQUE, Basque Foundation for Science, Bilbao 48013, Spain S Supporting Information *

ABSTRACT: Battery separators based on silk fibroin (SF) have been prepared aiming at improving the environmental issues of lithium-ion batteries. SF materials with three different morphologies were produced: membrane films (SF-F), sponges prepared by lyophilization (SF-L), and electrospun membranes (SF-E). The latter materials presented a suitable porous three-dimensional microstructure and were soaked with a 1 M LiPF6 electrolyte. The ionic conductivities for SF-L and SF-E were 1.00 and 0.32 mS cm−1 at 20 °C, respectively. A correlation between the fraction of β-sheet conformations and the ionic conductivity was observed. The electrochemical performance of the SF-based materials was evaluated by incorporating them in cathodic half-cells with C-LiFePO4. The discharge capacities of SF-L and SF-E were 126 and 108 mA h g−1, respectively, at the C/2-rate and 99 and 54 mA h g−1, respectively, at the 2C-rate. Furthermore, the capacity retention and capacity fade of the SF-L membrane after 50 cycles at the 2C-rate were 72 and 5%, respectively. These electrochemical results show that a high percentage of β-sheet conformations were of prime importance to guarantee excellent cycling performance. This work demonstrates that SF-based membranes are appropriate separators for the production of environmentally friendlier lithium-ion batteries. KEYWORDS: silk fibroin membranes, β-sheet conformations, environmental issues, battery separator, lithium-ion batteries

1. INTRODUCTION Lithium-ion batteries (LIBs) are one of the most relevant existent energy-storage technologies, commonly used in portable electronic devices, such as laptops and mobile phones, among others. Moreover, they are strongly being improved, targeting their use in terrestrial transportation applications, including passenger vehicles, such as electric vehicles.1−3 These batteries offer a number of attractive features. Apart from being light and cheap, they exhibit high energy density as other post-LIB systems (including lithium−metal batteries and lithium−sulphur batteries, among others) and power, long cycle life, and no memory effect.4,5 One of the major concerns in the area of energy storage is to produce sustainable and secure systems. Several technical issues of lithium-ion batteries still need further progress to further reduce their environmental impact and improve safety. With respect to these issues, the separator membrane with traditional organic liquid electrolytes © 2018 American Chemical Society

plays a key role considering security and environmental concerns, including its potential risk of explosion caused by short circuit.6,7 The separator is the component placed between the two battery electrodes, and its main function is to serve as a medium for the transfer of ions. It should exhibit good mechanical and thermal stability and be an electrical insulator with good ionic conductivity.8 Typically, the separator comprises a polymer membrane soaked in the electrolyte solution, that is, a liquid electrolyte, a solvent or mixture of solvents where the lithium salts are dissolved in.9,10 The properties of the separator membrane are influenced by different parameters, the most important being the degree of porosity, average pore size, Received: September 11, 2017 Accepted: January 25, 2018 Published: January 25, 2018 5385

DOI: 10.1021/acsami.7b13802 ACS Appl. Mater. Interfaces 2018, 10, 5385−5394

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ACS Applied Materials & Interfaces

specific capacity of 0.06 mA h cm−2, and its enzymatic degradation occurred over 45 days in the buffered protease XIV solution. In the present work, we demonstrate for the first time the potential of SF as an environment-friendly separator for lithium-ion batteries, thus enlarging the number of applications of SF in energy storage devices. SF was processed as a film membrane by solvent-casting as a sponge by lyophilization and as a fibrous membrane by electrospinning. The cycling performance of the as-produced battery separators was analyzed in depth, and their physical−chemical properties were characterized in parallel.

tortuosity, and thickness.8 The most widely used polymers for the development of separator membranes are poly(propylene) (PP),11 poly(ethylene oxide),12 poly(vinylidene fluoride) (PVDF),13−15 and their copolymers. Natural polymers, such as cellulose16 and lignin,17 are also beginning to be explored in this context. Considering the beneficial environmental impact that will result from the suppression of the use of synthetic polymers in the composition of separator membranes, it is highly desirable to replace them by natural polymers, while maintaining the excellent battery performance provided by their synthetic counterparts. Silk is considered to be one of the four most important available natural fibers.18 It is produced by arthropods, such as silkworms or spiders, scorpions, mites, bees, and flies. The most extensively produced silk is that spun from the silkworm of the domesticated Bombyx mori moth.19 The thread core of silkworm silk fibers is composed of a fibrous protein named fibroin which is surrounded by a glue-like protein named sericin that maintains the fibers together. Silk fibroin (SF) is an aqueous insoluble protein with more than 5000 amino acids and large molecular weight (200−350 kDa or more). SF possesses intrinsic features, including excellent mechanical properties, self-assembly, nontoxicity, or biodegradability, that largely exceed those of many polymers, either natural or synthetic.19 This unique biopolymer is largely known for its use in the textile and healthcare sectors (e.g., as suture). In the last decade, silk has been successfully processed in innovative ways and explored its application in the biomedical field, explaining why silk research is at present essentially focused on this domain application.20 However, it has been recognized in the last few years that reconstituted SF is extremely attractive for other technological areas, such as of optics, photonics, and electronics.21−23 In the field of solid-state electrochemistry, the potential of SF for electrochemical devices has been begun to be explored, and a few successful examples of devices including SF have been introduced in the last three years. Romero et al.24 incorporated interpenetrating networks made of SF and poly(pyrrole) (PPy) in the design of electrochemical actuators that are able to operate in a biological environment and generate forces comparable to those of natural muscles (>0.1 MPa) and that are stable to repeated actuation. Hou et al.25 reported hierarchical porous nitrogen-doped carbon nanosheets derived from biomass-derived natural silk with excellent characteristics for applications in the hybrid energy storage sector and in the area of renewable delivery devices with high power and energy density. Some of us23 developed green SFbased electrolytes exhibiting exceptional filmogenic properties, very high transparency, and suitable adhesion to glass substrates, which were employed in the fabrication of electrochromic devices (ECDs) with particularly good performance in terms of switching speed, cycling stability, and coloration efficiency. The produced ECDs were stable up to 5160 cycles, with a switching speed of about 15 s, a coloration efficiency up to 53.1 m2 C−1, and an optical modulation up to 5.5%. Jia et al.26 synthesized a partially biodegradable SF−PPy film cathode for a Mg−air bioelectric battery (biobattery) displaying a discharge capacity of 3.79 mA h cm−2 at a current of 10 mA cm−2 at room temperature and delivering a specific energy density of about 4.70 mW h cm−2. Very recently, Jia et al.27 fabricated a transient (biodegradable) thin-film Mg battery, encapsulated in SF and containing an electrolyte composed of SF and the choline nitrate ionic liquid. The battery offered a

2. EXPERIMENTAL SECTION 2.1. Materials. B. mori cocoons were supplied by APPACDM from Castelo Branco (Portugal). Sodium carbonate (Na2CO3, ≥99.5%), lithium bromide (LiBr, ≥99%), formic acid, and calcium chloride (CaCl2) were supplied by Sigma-Aldrich and used as received. Cellulose dialysis membranes (molecular-weight cutoff of 12 000−14 000 Da) were purchased from Medicell, International Ltd. All aqueous solutions were prepared using Millipore-Q water. PVDF (Solef 5130, Solvay), C-LiFePO4 (LFP, Phostech Lithium), carbon black (Super PC45, Timcal Graphite & Carbon), N,N′-dimethylpropyleneurea (DMPU, LaborSpirit), and 1 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC)−dimethyl carbonate (DMC) 1:1 vol/vol solution (Solvionic) were used as received. 2.2. SF Degumming. Degumming is the process of removing sericin from silk fibers to obtain pure SF. B. mori cocoons were cut into small pieces and degummed by boiling them for 30 min in a 0.02 M Na2CO3 solution. After that, the fibers were rinsed several times with distilled water, squeezed out to remove the excess water, and finally dried overnight in a fume hood. The obtained fibers were used in the following procedures. 2.3. Preparation of the SF Aqueous Solution. The SF aqueous solution was prepared according to the procedure reported elsewhere28 with some minor modifications. Briefly, fibers were dissolved in a 9.3 M LiBr solution at 60 °C for 4 h. The solution was subsequently dialyzed against ultrapure water until complete salt removal using cellulose dialysis tubing. The efficiency of the dialysis process was monitored by conductivity measurements. To remove impurities, SF aqueous solution was centrifuged twice at 8000 rpm at 4 °C for 20 min. The final concentration of SF (6.0 w/v %) was determined by measuring the dry weight of the solution. The asprepared SF solution was stored at 4 °C until further use. 2.4. SF Samples. To prepare the SF samples, three different techniques were adopted: solvent-casting, lyophilization (L), and electrospinning (E). The resulting materials were produced as a film (SF-F), a three-dimensional (3D) sponge scaffold (SF-L), and a 3D fibrous scaffold (SF-E). 2.4.1. SF-F. The SF aqueous solution was cast onto PP Petri dishes and then dried in a fume cupboard at room temperature during 72 h. The average thickness of the prepared films was 200 ± 30 μm. 2.4.2. SF-E. Degummed SF fibers were first dissolved in a 0.2 M CaCl2 formic acid solution at room temperature for 1 h. A film was prepared by casting this solution onto PP Petri dishes and then drying it in a fume cupboard at room temperature during 24 h. CaCl2 was removed from the film by immersion in water for 2 days. Water was changed four times per day. The complete removal of the salt was monitored by conductivity measurements. The as-prepared SF films were then dried in a fume cupboard at room temperature during 24 h and used to prepare a 8 wt % SF formic acid solution. This solution was drawn into a plastic syringe connected to a flux regulator. After an optimization process, a high voltage (15 kV) was applied between the syringe needle (0.5 mm inner diameter) and an aluminum foil, where the electrospun fibres were collected in the form of membranes with an average thickness of 13 ± 3 μm. A flow rate of 0.5 mL h−1 and a 15 cm needle-collector distance were used. 5386


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Figure 1. SEM images of the SF-E [(a) surface and (b) cross-section] and SF-L [(c) surface and (d) cross-section] separators. 2.4.3. SF-L. The SF aqueous solution was poured into a cylindrically shaped container and frozen at −20 °C (overnight) and −80 °C (6 h) prior to the lyophilization (−1.0 × 10−3 mbar, −86 °C). Lyophilization was performed in a Christ Alpha 2-4 LD Plus freeze dryer. Slices with an average thickness of 206 ± 25 μm were cut with a scalpel. 2.5. Characterization Techniques. Samples morphology were studied by scanning electron microscopy (SEM) using a FEI Nova 200 (FEG/SEM). Attenuated total reflection Fourier transform infrared (ATR/FTIR) spectroscopy spectra were obtained with a IRAffinity-1S FTIR spectrophotometer (Shimadzu, Izasa Scientific), resorting to an ATR accessory with a diamond crystal. The spectra were collected in the 4000−400 cm−1 range by averaging 64 scans at a resolution of 2 cm−1. Band deconvolution was performed using the residual procedure offered by the PeakFit software.29 More details about this procedure can be found in ref 23, in which Gaussian shapes were assumed and band frequency, bandwidth, and intensity were varied. Thermal properties of the samples were determined by differential scanning calorimetry (DSC, METTLER TOLEDO DSC 821e). The thermograms were acquired from 0 to 350 °C at a heating rate of 10 °C/min under argon purge in aluminum crucibles of 40 mL. The ionic conductivity (σi) of the SF separators (film, 3D sponge scaffold and 3D fibrous scaffold) was measured using Autolab PGSTAT-12 (Eco Chemie). The three SF samples filled with the liquid electrolyte (1 M LiPF6 in EC−DMC solution) were placed between two gold blocking electrodes, and complex impedance spectroscopy measurements were carried out at frequencies between 500 mHz and 65 kHz with an amplitude of 10 mV at room temperature. The ionic conductivity of the samples was determined by

σi =

d Rb × A

each SF separator was obtained as the average of the measurements performed in three different samples. 2.7. Electrolyte Uptake Value. The uptake value was obtained by immersing the SF separators into the liquid electrolyte and evaluated using the following expression

uptake =

W2 − W3 − Ws W1 − W3

(3)

where m0 is the mass of the dry SF separator and mi is the mass of the SF separator after immersion in the liquid electrolyte. 2.8. Preparation and Testing of the Cathode Electrode and the Half-Cell. The preparation of the LFP electrode is described in ref 30 based on the mixing of C-LiFePO4 (LFP), carbon black (Super P), and the PVDF binder in the DMPU solvent. Li/LFP half-cells into a Swagelok type were fabricated and assembled in a home-made Ar-filled glovebox. The half-cells contained lithium metallic as the counter/reference electrode, the LFP electrode film as the working electrode, and the different SF separators or a commercial separator (glass fiber membrane) soaked with the electrolyte (1 M LiPF6 in EC−DMC) as the separator. Galvanostatic measurements from 2.5 to 4.2 V at different scan rates (C/5 to 5C, C = 170 mA g−1) were carried out in the Li/LFP half-cell with a Landt CT2001A instrument. Electrochemical impedance spectroscopy was performed in the Li/LFP half-cells before and after cycling with an Autolab PGSTAT12 instrument at 10 mV between 1 MHz and 10 mHz.

3. RESULTS AND DISCUSSION 3.1. Morphology, Polymer Phase, and Thermal Properties of the SF Separators. The SF-F sample prepared by solvent-casting exhibited a nonporous texture (see Figure S1 in the Supporting Information). The lack of porosity demonstrated that SF-F could not adsorb and retain the liquid electrolyte and thus play the role of a battery separator. As a consequence, this material was discarded from subsequent studies. In contrast, the SEM images of the surface and crosssection of the SF-E and SF-L samples reveal the presence of highly interconnected porous structures with distinct morphology. As expected, SF-E was produced as a 3D scaffold composed of thin fibers with an average diameter of 256 ± 50 nm (Figure 1a,b). This sort of a 3D structure, characterized by high surface area to volume ratios and high porosity, is very attractive for application as a battery separator, as it enables fast

(1)

where d is the thickness, Rb is the bulk resistance, and A is the area of the sample. 2.6. Degree of Porosity. The degree of porosity of the SF separators was measured by the pycnometer method and calculated using the following relation

porosity =

m i − m0 × 100 m0

(2)

where Ws is the mass of the sample, and W1, W2, and W3 are the weight of the pycnometer filled with ethanol, the weight after placing the sample in the pycnometer with additional ethanol to complete the volume, and the weight after removing the sample from the pycnometer, respectively. The value of the degree of porosity of 5387


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Figure 2. (a) Results of the curve-fitting performed in the amide I region of the ATR/FTIR spectra of the separators. (b) Integral area fraction of the spectral components resolved in the spectral region. The different contributions to the amide I envelope are marked as random coil (R), β-sheets (B), α-helices (A), turns (T), and side chains.

to side chains, and those from 1637 to 1616 and 1705 to 1695 cm−1 are ascribed to ordered β-sheets, those in the 1655−1638 cm−1 range correspond to random coils, those in the 1662− 1656 cm−1 range are assigned to α-helices, and those between 1696 and 1663 cm−1 are attributed to turns.23,33 The profile of the amide I band of SF-F displays an intensity maximum at 1639 cm−1 (random coil range) and a shoulder at 1622 cm−1 (β-sheet range) (Figure 2a). Figure 2b reveals that in this material, the proportion of the random coil and ordered β-sheet conformations is practically the same (32 vs 29%, respectively). The amide I band of SF-E has an intensity maximum at 1648 cm−1 (random coil range). Figure 2b demonstrates that the dominant conformations of this sample are the random coils and turns. In the case of SF-L, the intensity maximum of the amide I band is downshifted to 1627 cm−1 (β-sheet range), and a shoulder is seen at 1643 cm−1 (random coil range). In this sample, 37% of the SF chains adopt β-sheet conformations, and 26% occur as random coils. The ATR/FTIR results unequivocally confirm that the processing method adopted to produce the three SF-based materials affected substantially the relative proportion of the chain conformations. Also, the ATR/FTIR spectrum of SF-L after immersion in the electrolyte solution indicate that the relative proportion of this conformation (β-sheet) increases in ∼7% (see Figure S2b of the Supporting Information). The thermal properties of the SF-based samples were determined by DSC. The DSC thermograms of the SF-based materials shown in Figure 3 demonstrate that all the samples exhibit a similar thermal behavior. The broad endothermic peak, centered at 64, 67, and 76 °C in the DSC curves of SF-F, SF-E, and SF-L, respectively, is associated with the release of water and/or solvent because water molecules strongly bind to silks and remain after thermal treatment.37 The glass transition, as well as crystallization and degradation peaks, is observed at characteristic temperatures of the SF-based materials.37−41 The glass transition temperature (Tg) occurs around 180, 169, and 180 °C for SF-F, SF-E, and SF-L, respectively. In addition, an exothermic peak attributed to β-sheet crystallization is evident

penetration of a significant amount of the guest electrolyte, an essential requirement to guarantee high ionic conductivity.31 Figure 1c,d shows that SF-L has a sponge-like 3D structure made of interconnected pores with an average size of ∼150 μm uniformly distributed along the surface (Figure 1c) and crosssection (Figure 1d). The higher magnification image of Figure 1c allows inferring that two types of open pores coexist in this scaffold: pores with an irregular shape and a diameter of ∼100 μm and smaller pores (diameter of ∼50 μm) circularly shaped. The various conformational arrangements present in the SF chains of the prepared materials were evaluated through the analysis of the ATR/FTIR spectra (Figure S2 of the Supporting Information). The results of the curve fitting performed in the amide I region and the integrated area fraction of the resolved components are represented in Figure 2a,b, respectively, for each sample. SF contains three crystalline polymorphs, so-called silk I, II, and III, which are usually considered as the main protein secondary structures.32,33 Silk I represents a metastable state between a partially ordered α-helix and the β-sheet structures, silk II is an antiparallel β-sheet structure, and silk III is formed at air−liquid interfaces.33,34 The ordered β-sheets of silk II, the most stable structures,33 are typically found in the silk fibers as raw material. In the case of regenerated SF, the polymer chains typically assume random conformations, which can be transformed into highly stable conformations through physical or chemical treatments.35 The secondary structure of SF is usually determined by an in-depth inspection of the amide I band, a characteristic absorption mode of the peptide backbone (Figure 2a). This mode is essentially associated with the carbonyl (CO) stretching vibration and is particularly sensitive to the specificity and magnitude of the hydrogen bonds. It is the most conformation-sensitive band to changes of the SF chains.23,33,36 To deduce the fraction of the various conformations present in the SF-F, SF-E, and SF-L samples, the amide I band of the ATR/FTIR spectra was deconvoluted based on the attribution proposed by Hu et al.33 Absorption bands in the frequency range of 1615 to 1605 cm−1 are assigned 5388


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concentration and on the relative fraction of α-helices and βsheets formed.42 The prevalence of one or another SF conformation depends on the organic solvent used.42 It was demonstrated that the swelling behavior of SF is influenced by restriction of β-sheet networks.43 The uptake process is illustrated in Figure 4b for SF-E. This behavior is similar to that of the SF-L sample. After the adsorption of the electrolyte, the color of the sample changed from white to transparent as a result of gelation. The process took place without mechanical fragmentation, and consequently, the sample remained mechanically stable (Figure 4b). The room-temperature ionic conductivity of the SF-E and SF-L separators was evaluated by complex impedance spectroscopy (Figure 5).

Figure 3. DSC curves of SF-F (black line), SF-L (red line), and SF-E (blue line) separators.

at 214 °C (crystallization enthalpy (ΔH) = 10.9 J g−1) for SF-F, at 197 °C (ΔH = 9.5 J g−1) for SF-E, and at 215 °C (ΔH = 21.4 J g−1) for SF-L. These values are in good agreement with previous reports.37,40,41 The enthalpy value associated with this thermal event increases with the increase of the fraction of βsheet conformations pointed out by FTIR/ATR (Figure 2). Beyond this event, all the SF-based materials immediately started to degrade, as substantiated by the presence of endotherms peaking at roughly the same temperature: 283, 282, and 286 °C for SF-F, SF-E, and SF-L, respectively. Considering the FTIR/ATR (Figure 2) and DSC (Figure 3) data, it is concluded that the content of ordered regions follows the order SF-E < SF-F < SF-L. 3.2. Degree of Porosity, Electrolyte Uptake, and Ionic Conductivity of the Membranes. The degree of porosity and the electrolyte uptake values determined for the 3D porous scaffolds, that is, SF-E and SF-L, may be deduced from Figure 4a. The differences observed can only be explained in terms of morphological arguments (Figure 1). The degree of porosity of SF-E (68%) is slightly higher than that of SF-L (60%) but lower when compared to commercial separators (see Table S1 of the Supporting Information). This trend is correlated with that of the uptake value (1738 and 1111%, respectively, after 15 s of immersion in the electrolyte). It must be noted that these values are associated, not only with the extent/type of the micropores, but also with the SF/ electrolyte interactions. In this respect, the accessibility and interconnectivity of the pores in the SF-E sample allows accommodating a larger amount of the electrolyte for a similar degree of porosity because of its larger swelling ability/capacity. It is known that the dissolution of SF-based materials in organic and mixed aqueous−organic salt systems depends on the salt

Figure 5. Nyquist plots at 25 °C for the SF-L (red symbols) and SF-E (black symbols) separators. The lines drawn are guides for the eyes.

The Nyquist plot of Figure 5 for both SF-based membranes is characterized by a straight line in the whole frequency range,44 which is attributed to the high uptake value and represents the electrode/electrolyte double layer capacitance behavior.45 The ionic conductivity value was calculated using eq 1, and the resistance of both samples was obtained from the intercept with the Z′ axis. The ionic conductivity values measured for SF-E and SF-L were 0.32 and 1.00 mS cm−1, respectively, which is lower when compared to commercial separators (see Figure S3 of the Supporting Information). As the degree of porosity is similar in both cases and the electrolyte uptake is larger for the SF-E sample, we believe that the higher ionic conductivity of SF-L can only be related to different β-sheet conformation content, which is higher for SFL. Thus, the planar conformations of the polymer β-sheets apparently promotes higher ionic conductivity because of the

Figure 4. (a) Degree of porosity and uptake value of SF-E and SF-L separators after immersion in the electrolyte. (b) Swelling process of SF-E after electrolyte uptake. 5389


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Figure 6. Charge−discharge profiles for the SF-L separator (a). C/2 and 2C rates (b), rate performance as a function of the cycle number (c), and capacity retention (d) during the discharge process for SF-E and SF-L separators.

Figure 6 shows the electrochemical performance of the different half-cells for scan rates from C/5 to 5C. Figure 6a reproduces the charge−discharge profile of the half-cell with the SF-L separator in the 5th cycle, which is characterized by two voltage pseudoplateaus that represent the reversible charge (lithium removal)/discharge (lithium insertion) process, that is, the Fe2+/Fe3+ redox reaction of the C-LiFePO4 spinel, independently of the C-rate.50 Figure 6a also reveals that the cycling profiles decrease with increasing C-rates except for the C/5-rate. This behavior is ascribed to the Ohmic polarization effect, and the lower discharge value at C/5 is due to the formation of the solid electrolyte interface (SEI) during the first cycles.45,51 For the SF-L separator (Figure 6a), the discharge capacity values are 107, 126, 114, 98, and 38 mA h g−1 at rates of C/5, C/2, C, 2C, and 5C, respectively, indicating excellent electrochemical values at C-rates above C. Figure 6b shows the 5th charge−discharge curves at C/2 and 2C-rates for SF-E and SF-L. The voltage profile is very similar for both materials, the discharge capacities of SF-E and SF-L being 108 and 126 mA h g−1 at the C/2-rate and 54 and 99 mA h g−1 at the 2C-rate. Further, the rate performance for both SF separators is depicted in Figure 6c, demonstrating a better rate capability for the SF-L separator membrane because of the higher discharge capacity at the various C-rates. In addition, Figure 6c also shows that in terms of cycling performance, it is very stable as a function of the cycling number, independently of the C-rate. The excellent rate capacity of the SF-L separator is attributed to its higher ionic conductivity value, which is correlated in turn with a high fraction of β-sheet conformations, as shown in Figure 8, and also to a decrease in the interfacial resistance between the separator and electrodes related to the compact morphology of this sample.52 The capacity retention in the discharge process (capacity normalized with respect to the C/5 rate), as a function of the

interaction established between the lithium ions and the dipole moments formed in the crystalline regions as a result of the extensive hydrogen bonding interactions the peptide chains of SF are involved in.46 This issue will be discussed in more detail below. It must be emphasized that both samples display ionic conductivities higher than 0.1 mS cm−1 which is the minimum ionic conductivity value required for lithium-ion battery separator applications.47 Two other important parameters to characterize battery separators are the tortuosity (τ) value, which represents the ratio of the mean effective capillary length to separator thickness and describes the average pore connectivity of the separator,31 and the MacMullin number (N M). These parameters are calculated, respectively, using the following equations48,49 σ NM = 0 σeff (4) where σeff = σ0

ϕ τ2

(5)

where σeff is the ionic conductivity of the separator and the electrolyte pair, σ0 is the ionic conductivity of the pure liquid electrolyte, and ϕ is the porosity of the separator. The values found were 5 and 3 for SF-E and SF-L, respectively, thus very similar, indicating high pore connectivity that results in faster ion transport properties. The NM values are 36 for SF-E and 12 for SF-L, indicating that SF-L shows a relatively lower resistivity with respect to SF-E and therefore that it should exhibit a better battery cycling performance. 3.3. Battery Performance. The electrochemical performance of SF-E and SF-L as battery separators was investigated in Li/C-LiFePO4 half-cells fabricated using C-LiFePO4 as a cathode and lithium foil as counter and reference electrodes. 5390


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Figure 7. Schematic representation of the interaction between lithium ions and the dipole moments (red arrows) of the β-sheet structure of the SFbased materials.

Figure 8. (a) Capacity and CE for the half-cells incorporating the SF-L separator when cycled at C and 2C-rates and (b) discharge capacity value for over 50 cycles at 2C for SF-E, SF-L, and glass fiber commercial membranes.

C-rate for both separator membranes, is represented in Figure 6d. A progressive decrease of the capacity retention value with the increasing C-rate is observed except for SF-L at C-rates below C, this behavior being attributed to a diffusion phenomenon taking place within the cathode electrode and the separator.53 Moreover, the stable SEI layer of SF-L results is in good cycling performance because of steady ionic transportation for the high C-rate.54 The degree of porosity, electrolyte absorption, and the degree of crystallinity of the two separators are practically the same, being nevertheless the ionic conductivity the main parameters with relevant variations. Typically, in polymer electrolytes, the ionic conductivity (migration and transport of ions) is determined by large-scale segmental movements of the polymer chains in the amorphous regions.55 In the present case, the presence of crystalline regions in the separator scaffold was beneficial, as it enhanced ion movement mobility as a result of the efficient interaction established between the lithium ions and the dipole moments of the antiparallel β-sheet structure (Figure 7), explaining the higher ionic conductivity exhibited by the SF-L with respect to SF-E. As already pointed out, SF has a semicrystalline fibrillar character. Its ordered regions correspond to an antiparallel βsheet structure with a monoclinic unit cell, as illustrated in Figure 7.56 This β-sheet structure gives rise to an internal polarization which is responsible for the piezoelectric effect of SF.57 In fact, it was demonstrated that β-sheet crystallinity is one of the prerequisites that must be fulfilled to guarantee a strong piezoelectric effect in silk.57 Clearly, in the present case, this local electric field facilitated motion of the lithium ions58

via the interaction of the charge carriers with the dipole moments of the β-sheets. This efficient β-sheet-assisted hopping mechanism enhanced ion migration through the separator, leading to the increase of ionic conductivity.59 Figure 8a shows the cycling performance and Coulombic efficiency (CE) for SF-L at C and 2C-rates during 50 cycles, in which good cycling stability is observed for all cycles. Furthermore, the discharge capacity value after 50 cycles at 2C is 81 mA h g−1, corresponding to 72% of the capacity retention. Also, for this sample, Figure 8a shows that the CE is about 100% and remained stable upon cycling, which is related to the reversibility of the process, that is, the movement of active species through the separator. Figure 8b shows the cycling performance of the SF-E and SFL membranes at 2C for over 50 cycles and its comparison with a glass microfiber separator. After 50 cycles, the discharge capacity values were 81, 53, and 18 mA h g−1 for SF-L, glass microfiber, and SF-E, respectively, indicating the excellent electrochemical performance of the SF-L sample due to its high ionic conductivity. Table 1 summarizes the electrochemical results of the SF separator membranes after 50 cycles at C and 2C-rates. The SF separator with the best electrochemical performance was SF-L. At the 2C-rate (charge and/or discharge process in half an hour), its capacity retention and capacity fade are 72 and 5%, respectively, demonstrating high rate capability. Considering the results of Table 1, we may state that a lithium-ion battery containing SF as the separator can cycle at high rates with a very controlled capacity decay with a low resistance value (Figure 9). 5391


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lyophilization. The ionic conductivity value measured after electrolyte uptake was higher for the SF-based membrane produced by lyophilization (SF-L) than for that fabricated by electrospinning (SF-E). As the degree of porosity is similar and the uptake is higher for the SF-E sample, the higher ionic conductivity exhibited by SF-L is attributed to the higher βsheet conformation content. Cathodic half-cells with the SF-L samples exhibited a higher capacity value and enhanced cycling performance than a battery incorporating a commercial separator. After 50 cycles at 2C, the SF-L separator showed a discharge capacity value of 81 mA h g−1, corresponding to a capacity retention of 72%. The electrochemical results suggest that the SF-based separators with a high fraction of β-sheet conformations have excellent cycling performance, thus offering excellent prospects for the fabrication of separators of environmentally friendlier and more sustainable lithium-ion batteries.

Table 1. Discharge Capacity Values, Capacity Fade, and Capacity Retention for the SF Membranes at C and 2CRates SF-L first discharge capacity (±3)mA h g−1 50th cycle discharge capacity (±3) mA h g−1 capacity fade: 2−50 cycles (%) capacity retention after 50 cycles(%)

SF-E

C

2C

C

2C

117 107 9 95

85 81 5 72

91 80 12 68

32 19 40 16

It is of interest to refer that battery performance for the SF-L separator is similar or even better than those of green polymer membranes, such as lignin,17 aramid,60 and cellulose,61 used as battery separators, but presents several additional advantages with respect to these polymers, such as piezoelectricity, biocompatibility, and degradability effect, exceptional mechanical properties, multiple variety of material formats, and so forth. Finally, to evaluate the resistance changes of the cathodic half-cells based on the both separators, the ac impedance spectra were obtained before cycling in a freshly assembled cell (Figure 9a) and after (Figure 9b) cycling. Figure 9 shows the Nyquist spectra with a semicircle in the high frequency region that represents the total resistance (sum of SEI resistance and the charge-transfer resistances process) corresponding to the migration of lithium ions between the electrode and the electrolyte interface and a straight line in the low frequency regions that represents the diffusion of lithium ions in the active material of the cathode electrode.30 Figure 9a shows that the total resistance value of SF-E before cycling was 2000 Ω, a value which is slightly lower than that of SF-L (3375 Ω). After cycling, the resistance values of SF-E and SF-L were 6679 and 3313 Ω, respectively (Figure 9b). Thus, the resistance increased after cycling for SF-E as a result of a significant increase of the interfacial resistance, leading to reduced lithium ion diffusion. For SF-L, its resistance value remained practically constant, indicating better interface stability which is at the basis of the excellent cycling performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13802. Morphology of the SF-F sample; conformational state of the SF chains in the SF-based materials; and degree of porosity, uptake value, morphological and electrical properties of the commercial separator (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.F.P.P.). *E-mail: cmscosta@fisica.uminho.pt (C.M.C.). *E-mail: [email protected] (S.L.-M.). ORCID

Rui F.P. Pereira: 0000-0001-7279-5728 Renato Gonçalves: 0000-0001-9763-7371 Carlos M. Costa: 0000-0001-9266-3669 Maria Manuela Silva: 0000-0002-5230-639X Author Contributions

4. CONCLUSIONS New environmentally friendly separators based on SF were prepared by solvent-casting, electrospinning, and lyophilization. Porous 3D scaffolds with different porous morphologies, high degree of porosity, excellent thermal stability, and good electrolyte uptake were produced by electrospinning and

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

Figure 9. Impedance spectroscopy of the cathodic half-cell incorporating SF-E and SF-L separators: before (a) and after (b) cycling. 5392


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ACKNOWLEDGMENTS This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013, UID/QUI/00686/ 2013 and UID/QUI/00686/2016. The authors thank FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under the projects PTDC/CTM-ENE/ 5387/2014 and UID/CTM/50025/2013, PEst-OE/QUI/ UI0616/2014, and LUMECD (POCI-01-0145-FEDER016884 and PTDC/CTM-NAN/0956/2014), grants SFRH/ BPD/87759/2012 (R.F.P.P.), and SFRH/BPD/112547/2015 (C.M.C.). The authors acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R (AEI/FEDER, UE) and from the Basque Government Industry Department under the ELKARTEK program.

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